Alveolar macrophages are immune cells located in the alveoli of the lungs and are among the first cells to encounter microorganisms that enter the lower respiratory tract. Under normal conditions, alveolar macrophages maintain lung homeostasis by engulfing foreign particles and dying cells while releasing only small amounts of inflammatory molecules. This balance matters in the lungs because excessive inflammation damages tissue and can impair gas exchange.
The properties of alveolar macrophages are largely determined by their environment. High oxygen concentrations, pulmonary surfactant, and local signaling molecules influence their function. One key factor is granulocyte-macrophage colony-stimulating factor (GM-CSF), a protein required to maintain normal macrophage numbers and function. The importance of this protein is evident in people with impaired GM-CSF signaling: they may develop pulmonary alveolar proteinosis, in which macrophage numbers and function are reduced, and they become more susceptible to pulmonary infections.
The origins of alveolar macrophages remain controversial. The alveolar macrophage population may arise from several sources:
- Early yolk sac-derived macrophages are later largely replaced by cells derived from fetal liver monocytes.
- Yolk sac-derived macrophages may persist into adulthood.
- During inflammation, the macrophage population may also be replenished by cells derived from bone marrow monocytes.
Alveolar macrophages recognize signs of infection and tissue damage through pattern recognition receptors (PRRs) and initiate a protective response. However, pathogens can suppress or circumvent this mechanism. If they reach the lower respiratory tract and disrupt macrophage function, infection may be accompanied by efficient pathogen replication, severe inflammation, and lung tissue damage.
In response to pathogens, alveolar macrophages produce type I interferons (IFN-α and IFN-β). IFN-I plays different roles in determining alveolar macrophage responses to acute viral and chronic bacterial infections and, consequently, substantially influences disease course and outcome.
Macrophage Origin Determines Their Fate During Lung Inflammation
During lung inflammation, the macrophage population changes rapidly: signaling molecules, including IFN-I and the chemokine CCL2, recruit monocytes from the bone marrow, substantially expanding the myeloid leukocyte pool.
After acute lung injury caused by influenza A virus or lipopolysaccharide (LPS) administration, monocyte-derived macrophages gradually die through Fas-dependent apoptosis. At the same time, the number of resident alveolar macrophages changes very little. Other studies have confirmed that resident alveolar macrophages have an extended lifespan and can maintain their population independently of bone marrow monocytes.
Thus, macrophage fate during inflammation depends on their origin and the nature of the immune response. IFN-I is the main regulator of macrophage fate and function.
Effects of Type I Interferons During Viral Infections
During influenza A virus (IAV) infection, alveolar macrophages are a major source of type I interferons. IFN-I initiates antiviral programs in infected and neighboring cells, limits viral replication and spread, and helps initiate the adaptive immune response. Influenza virus, in turn, can suppress this protective mechanism.
Although respiratory epithelial cells are the primary site of influenza A virus replication, numerous studies show that IAV also infects macrophages and replicates productively in these cells. A recent study showed that CD169+ splenic macrophages infected with vesicular stomatitis virus (VSV) selectively reduce the IFN-I response to enhance viral replication and make viral antigens available to initiate and maintain antigen-specific T- and B-cell responses. Thus, tight regulation of IFN-I production by macrophages after infection may serve to control viral replication while simultaneously promoting activation of the adaptive immune response.
IFN-I production is also regulated by eicosanoids – lipid signaling molecules. For example, PGE2 suppresses type I interferon production and therefore increases susceptibility to influenza virus. Thus, the outcome of pulmonary infection depends on how both the host and the pathogen regulate the eicosanoid–interferon axis.
IFN-I regulates the influx of myeloid cells into infected lungs. During influenza, IFN-I produced by alveolar macrophages promotes monocyte release from the bone marrow, thereby supporting immune protection. When the IFN response is impaired, other populations of monocytes and neutrophils accumulate in the lungs, increasing tissue damage and being associated with higher mortality.
Immunoregulatory Properties of Type I Interferons
The effects of IFN-I depend on the stage of infection. During the early stages, IFN-I helps limit viral replication. In chronic infection, the body may use persistently produced IFN-I to activate anti-inflammatory responses that limit immune-mediated tissue destruction.
Reference
Alveolar macrophages and type I IFN in airway homeostasis and immunity
